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Ulduar consists of a huge complex of Titan origins, reflected in both the beautiful architecture and denizens of the place. Added in Patch 3.1, the raid features optional bosses and was the first to offer true "hard mode" difficulties and rewards for certain bosses. Some mechanics were new, such as General Vezax's Aura of Despair (although it has yet to be used since then), while Yogg-Saron gives players the opportunity to witness significant events in Azeroth's past and present. Ulduar also presented the first legendary in Wrath of the Lich King, the healing mace Val'anyr, Hammer of Ancient Kings.

And then there isAlgalon the Observer. Algalon, prior to Patch 4.2, required at least one raid member to have either a Celestial Planetarium Key (10-man) or Heroic Celestial Planetarium Key (25-man). Players are then treated to a neat opening sequence for the door which leads into Algalon's room; a very beautiful room that only improves as the encounter begins. He was also the first ever boss that:

Was "hard mode only"

Had a 1-hour time limit in which players could attempt to defeat him for that lockout period (removed in Patch 4.0.3a and which has not been repeated to this day)

Accordingly gave exclusive lore and rewards

It is because of all this and more that players generally rank it alongside Karazhan as being one of the best raid instances ever made. And it is probably because of those factors that make it all the more bitter that Ulduar's duration only lasted just shy of five months -- the shortest length of any raid tier ever. If there's any consolation, it has left as its legacy a warning to Blizzard to not repeat that tragic mistake.